1. Field of the Invention
The present invention relates to a jitter equalizer for a digital transmission filter limiting an occupied frequency bandwidth of a transmission signal to achieve the most efficient use of frequency in the digital transmission. In particular, the invention is concerned with a digital technique for achieving minimal occurrence of jitter, overshoot and undershoot in an output waveform in the filter for digital transmission.
2. General Description of Prior Arts
In filtering a digital transmission signal by a known digital filter, even if its group delay characteristic is compensated almost completely, tails of adjacent pulse outputs before and after an object pulse output frequently lead to distortion of the object pulse output. This distortion causes uneven zero-crossing of the output pulse waveforms and inevitably results in phase vibration error, that is, jitter. The overshoot and undershoot of output pulse waveforms in the filter are made due to the tails of the adjacent pulses when a sequence of pulses with the same logic level are provided.
Generally, in a digital transmission system, it is impractical from the view point of efficient frequency usage, to transmit an original digital pulse with comparatively broad bandwidth, without any modification or alternation thereto. Thus, so long as it has no significant effect on digital signals to be transmitted, the transmission should be preferably carried out with minimal bandwidth limitation. This will be effectively achieved by a filter limiting the bandwidth of transmission signals to a specified bandwidth, so that a most numerous, plurality of channels can be transmitted within a frequency range of the specified bandwidth, and the largest amount of information can be transmitted without occurrence of interference between each of the adjacent channels.
Filtering of the transmission signals is generally achieved by two known arts. One is to use a filter for bandwidth limitation at a final output stage after modulation of digital information. The other is to carry out modulation of a carrier signal after prefiltering of input digital pulses. The former system requires a filter having a very narrow bandwidth at a high frequency, which is considerably difficult to construct. Thus the latter system has been mainly used in current digital transmission (see U.S. Pat. No. 4,644,565 of Jongsoo Seo). In the present invention, therefore, an equalizer for filters according to the latter system will be specifically described hereinafter.
During filtering of a signal waveform in a filter, the phase delay characteristic within a passband changes non-linearly with its frequency variance. Therefore, most filters in digital transmission systems have therein a group delay equalizer to compensate non-linearity of phase delay according to a frequency band of the filter, so as to prevent distortion of information resulting from inter-symbol interference of transmission signals.
However, even if the filter is one completely compensated in its group delay as above described, referring to FIGS. 1A and 1B, wherein the transfer characteristic on a pulse waveform of period (Ts) in the filter is shown, a main lobe of the output response is two times as wide as the period (Ts), and some tail components thereof are left. These tail components affect adjacent pulses such as preceding pulses or following pulses of the original pulse. FIGS. 1C and 1D respectively show waveforms of a base clock signal S.sub.0 and a random Non-return to Zero (NRZ) digital data pulse signal S.sub.1 synchronized with the base clock S.sub.0. FIG. 1E shows output response to input of the random NRZ digital data pulse signal S.sub.1 in the filter, wherein a waveform D.sub.3 corresponds to an ideal output response. However, owing to the above described transfer characteristic leaving such tail components in the filter, various pulse responses including tail components such as dotted curves D.sub.2 in FIG. 1E are subtracted and added in amplitude to each other to produce an undesirable output waveform. Accordingly, this causes the distortion of output response in the filter, as shown by the waveform D.sub.1, and as a result, the zero-crossing points(X) in the output waveform become uneven.
FIG. 2A shows an ideal eye-diagram observed on an oscilloscope when the above output waveform of the filter is synchronized with a time axis of the oscilloscope by the base clock S.sub.0 of FIG. 1C. The ideal eye-diagram shows no distortion. However, FIG. 2B shows an actual eye-diagram observed on the oscilloscope, wherein a plurality of overlapping waveforms in overshoot or undershoot show up owing to the above described tail components. This fluctuation of the zero-crossing timing is referred to as jitter distortion. Further, the tail components lead to the overshoot and undershoot in amplitude as seen in FIG. 2B. The larger the amount of the bandwidth limitation in a filter, the larger these jitter distortion, overshoot and undershoot phenomena become.
When a symbol timing recovery operation extracting a clock synchronized in phase with that of the transmission end is carried out to make precise the demodulation of data at a receiving end, the jitter distortion severely affects the reproduction. In the clock recovery operation, as the phase of the transmission signal is estimated, in most cases, on the basis of zero-crossing time of the received signal, the fluctuation of zero-crossing timing from the transmitting end of the transmission signal makes unstable the phase of the reproduced clock, which inevitably leads to decreased error performance of the receiver. To overcome this problem in a conventional system, a phase locked loop has been used in a receiver to compensate the jitter distortion, the phase locked loop being comparatively less sensitive to the jitter fluctuation in transmission. However, since, the coverage of such compensation by the phase lock loop requires very narrow loop bandwidth characteristics and hardware implementation is very difficult, the above mentioned problem remains still unresolved in the digital transmission system with bandwidth limitation.
The aforementioned overshoot and undershoot phenomena in amplitude is mostly applied to a power amplifier of a transmitter and leads to saturation of the power amplifier, which results in an increase of unnecessary bands in transmission.
U.S. Pat. No. 4,339,724 to Feher discloses a non-linear filter for minimizing such jitter distortion, overshoot and undershoot phenomena in amplitude as described above. However, this has still a drawback in that its bandwidth limitation is carried out in a relatively small range and further, is not variable over the specified small data rate range.